Cellulosic film and process for preparing same



Patented Aug. 29, 1939 UNITED STATES PATENT OFFICE CELLULOSIC FILM AND PROCESS FOR PRE- PARING SAME poration of Delaware No Drawing. Application January 4, 1936,-

Serial No. 57,633

16 Claims.

This invention relates to flexible, cellulosic pellicles and particularly to such pellicles which have been rendered resistant to change in dimensions caused by vibration in humidity condi- 5 tions.

Cellulosic pellicles which are obtained by coagulation or precipitation from aqueous or alkaline aqueous dispersions of cellulose or cellulose derivatives, as for example, pellicles of re- 10 generated cellulose, glycol cellulose, 'cellulose glycolic acid, slowly esterified or etherified cellulose such as lowly etherified methyl and ethyl cellulose and lowly esterified cellulose acetate, are particularly useful as wrapping materials. It

15 is essential, however, that these pellicles be flexible and further that this flexibility be maintained. Cellulosic pellicles of this character seem to depend on the presence of moisture to impart flexibility and the softeners which have been used 20 heretofore have been selected because of their hygroscopic nature. It has been considered necessary to have the softener assist, by virtue of its hygroscopicity, in the retention of a suitable amount of moisture although the softener has 25 possessed, also, a certain degree of lubricating or plasticizing action so that the pellice does not become too brittle when practically all of the moisture has been removed. In the absence of a softener other than water, these pellicles become so brittle when dried as to be substantially useless as wrapping tissues. In the presence of the hygroscopic softeners of the prior art, the loss of moisture is retarded and generally speaking, the flexibility is maintained by the moisture pres- Regenerated cellulose pellicles, (for convenience the invention will be described in terms of this species) are quite sensitive to changes in moisture content, not only as regards flexibility, but also 40 as regards dimensions. Increase in moisture content causes a swelling of the cellulosic structure,

while decrease in moisture content causes a shrinking apparently due in part to collapse of the cellulose micelles 'and in part to the loss of water molecules from between the micelles.

Regenerated cellulose wrapping tissues are subject,- therefore, to .two major defects which develop simultaneously and which are primarily dependent on the softener, namely embrittlement 50 and deformation. Previous attempts to control or prevent embrittlement, however, have not been successful in controlling the deformation for, as

mentioned above, the softeners selected have been highly hygroscopic and have really resulted in 5 greater sensitivity to moisture conditions, thereby increasing the expansion and contraction of wrapping tissues or similar pellicular structures.

Thus it is that great care is required in wrapping boxes in regenerated cellulose pellicles to provide for these conditions. For example, if a 5 cereal box is wrapped in regenerated cellulose sheeting (prepared in accordance with the prior commercial methods and softened with glycerin which is the usual hygroscopic softener) and set aside for storage, it may be subjected to a variety of humidity conditions before it reaches its ultimate consumer. During this time, if the humidi-ty is high, the regenerated cellulose may expand until the wrapper becomes loose around the box and in some cases even baggy and wrinkled. On the other hand, if the humidity is low, oss of moisture from the regenerated cellulose will cause the wrapper to contract and this maycause buckling of the box walls, or if the box issufficiently rigid, the wrapper itself may burst. Thus, under such a variety of humidity conditions, packages wrapped in regenerated cellulose sheeting may have an unsightly and undesirable appearance as the result of the deformation of the regenerated cellulose. Obviously, this defeats the very important purposes of regenerated cellulose wrappers which are intended to protect and beautify articles wrapped therein. To overcome the troublesome deformation, wrapping machinery has been designed to allow for a certain slack or looseness in the wrap. This obviously will eliminate the effects of contraction, butcannot help the expansion effects; indeed, it makes them worse. The use of moistureproofed regenerated'cellulose pellicles does not eliminate the trouble because the application of a moistureproofing coating retards but does not prevent the deformation, which latter is in the ultimate not appreciably affected. These practical means do nothing more than attempt to 40 make the best of the situation and make use of regenerated cellulose sheeting as it is available. No attempt is made to change the inherent properties of the cellulosic material and so provide a wrapping which will have improved properties and characteristics.

It has been found'in the manufacture of regenerated cellulose pellicles that the degree of deformation is, at least in part, due to the orientation of the cellulosic micelles. Thus, if a sheet of regenerated cellulose is made in such a way as to provide substantially uniform tension in all directions, thedegree of deformation will be substantially the same in all directions. Practically, however, regenerated cellulose pellicles are made in a continuous process wherein the pellicle, after its formation, is drawn through a series of treating and purifying baths and finally over a series of drying rolls. It is necessary, therefore, in order to advance the pellicle through the cycle of treatment, to apply a certain amount of tension on the pellicle in the direction of travel, usually referred to as the machine direction. This results in an appreciable difference in the degree of deformation if measured in the machine direction, as compared to the degree of deformation if measured at right angles to the machine direction, i. e. the transverse direction. However, in any given process the conditions are such that the degrees of deformation in the machine and transverse directions usually bear a sufficiently constant relationship so that for test purposes it is usually sufficient to determine the degree of deformation in one direction only, usually the machine direction.

For the purposes of this specification the deree of deformation, which may be called simply deformation, is the per cent change in length of a cellulosic pellicle as measured in the machine direction in accordance with the following procedure: Strips of material are allowed to come to equilibrium with an atmosphere of substantially relative humidity at a temperature of 35 C. and their length accurately measured. The strips are then allowed to reach equilibrium in an atmosphere of substantially 0% relative humidity at the same temperature and their length again accurately measured. The difference in length divided by the length as originally measured multiplied by gives the per cent deformation over the given humidity range at 35 C. and figures so obtained are conveniently called the deformation. Thus, a sample strip having an original length of 10 inches and a contraction in length of 0.42 inch would be said to have a deformation of 4.2.

It is the object of this invention to provide means for reducing the deformation of cellulosic pellicles. It is also an object of the invention to effect simultaneous softening and reduction of deformation. It is a further object to provide a method whereby the aforementioned objects may be accomplished in an economically feasible manner and without entailing essential modification of apparatus customarily used in the manufac- I ture of such pellicles.

More specifically, it is the object of the invention to provide meansfor the production of cellulosic pelllcles which will show less deformation than the cellulosic pellicles presently commercially available.

Specifically, it is the object of the invention to produce a pellicle of regenerated cellulose suitable for use as a wrapping tissue which will show a degree of deformation of not in excess of 3.0 when tested in accordance with the method described above. This object also includes the production of a regenerated cellulose pellicle which is combined with a softening material and which shows a deformation not in excess of 3.0.

The above and other objects of the invention may be accomplished by impregnating into the cellulosic pellicle, preferably while the latter is in a wet or gel state, an appreciably watersoluble, high-boiling (i. e. relatively non-volatile at ordinary temperatures and pressures), .lim- I itedly hygroscopic material which is stable, essentially colorless and odorless, preferably non-toxic and which is preferably also a softening agent for the cellulosic material. The extent of the reduction in deformation which may be obtained depends on the particularimpregnant used' and to a certain extent on the amount employed.

The term limitedly hygroscopic is intended to include substances which will absorb 180% of their weight of water when exposed alone in a thin layer to an atmosphere of substantially 95% relative humidity at a temperature of 25 C. over a period of hours. The test for hygroscopicity is carried out as follows: A small sample of about 0.52.0 grams of thoroughly dried material is spread evenly over the bottom of a weighting bottle (conveniently about 2%" in diameter and 1 deep) and the weight of the sample accurately measured. The open weighing bottle is then placed in a chamber in which an atmosphere of substantially 95% relative humidity and a temperature of 25 C. is maintained. The humid atmosphere may be maintained conveniently by means of a sulfuric acid solution (9 parts of water to 1 part sulfuric acid) contained within the chamber. The sample, after 120 hours exposure, during which time the material is occasionally agitated as by gently tipping the container to cause the material to flow over the bottom of the weighing bottle, is reweighed accurately and the percentage increase in weight based on the original weight of the sample repre-- sents the hygroscopicity of the material.

'Based on the method described, those substances suitable for use in the practice of the invention will have a hygroscopicity of 1 to 80.

Generally speaking, the test of water-solubility is applied first to determine the suitability of substances for use in the practice of the invention. If the substance is at least 1% and not more than 40% soluble in water at 20 C. its utility is indicated. In the case where the solubility is more than 40%, the hygroscopicity should be determined and in the event that this value should be above 80, the substance will be useless for the purposes of the invention.

The third physical criterion relates to volatility. The substances must be high-boiling, that is, substantially non-volatile at ordinary temperatures and pressures, and should be preferably though not necessarily liquids. Obviously, if substances of appreciable volatility were to be used, they would eventually disappear from the cellulosic pelllcles rendering them thereby brittle, fragile and unsuitable for use as wrapping tissues or other applications. It has been found that substances of appropriate water-solubility and hygroscopicity which have a boiling point of C. or higher, at a pressure of 12 mm. of mercm'y and preferably of C. or higher (at 12 mm.) will be suitable for use.

In the preferred form of the invention substances having limited hygrosoopicity and appropriate boiling-point and which are at least 4% soluble in water at 20 C. will be used to impregnate the cellulosic pellicles to effect a reduction in deformation. Substances of this character may be introduced into the pellicles by means of a simple water solution. In the case where it is desired to use a substance which is less than 4% soluble, it may be convenient to increase the concentration by adding a. watermiscible organic solvent such as methanol, ethanol, acetone or thelike although in no case should more than 25% by volume of such solvent in the water-solvent mixture be necessary. Substances which require more organic solvent are unsuited for the purposes of the invention. Inasmuch as the use of a portion of organic solvent entails additional expense, it is desirable to use those substances which are at least 4% soluble although the less soluble substances will find many useful applications.

Although the principal objective of the invention relates to the'reduction of deformation, it is apart of the invention to provide also a flexible pellicle. This can be done frequently by proper choice of the impregnating agent so that itwill be at one and the same time a softener and a deformation reducing agent. For simplicity, such an agent may be termed a nondeforming softener. Obviously, other properties may be combined with these in certain agents, which will have particular advantage.

Among the non-deforming agents which are especially suitable for use there'are a number of esters derived from keto-acids. These esters may be synthesized by the esterification of monoor polyhydric alcohols with carboxylic acids which contain ketone groups in their molecular structure. As the alcohol portion of the ester, mono-, di, or polyhydric alcohols, preferably aliphatic, although alcohols containing cyclic or aromatic groups, which may optionally contain one or more ether groups, may be used. Preferably primary alcohols are used although secondary and tertiary alcohols are contemplated where desirable. Such alcohols as ethylene glycol, propylene glycol, diethylene glycol, ethoxyethanol, methoxyethanol, glycerol, or the like, are illustrative of those which may form a portion of the ester. Keto-acids such as levulinic, pyruvic, benzoyl benzoic, acetoacetic, to mention only a few, constitute the acid residue in the keto-acid esters of the invention.

Although the preparation of the esters is not a part of the invention, it should be understood that only simple non-resinous organic keto-acid esters are useful in the practice of the invention.

Accordingly, care must be taken to avoid such combinations of acids and alcohols as will produce resinous substances unless appropriate precautions are observed to prevent resin formation.

Generally speaking this means that suitable esters may be obtained by esterifying monohydric or polyhydric alcohols with monobasic acids; polybasic acids with monohydrlc alcohols; but not polybasic acids with polyhydric alcohols unless in this latter case the formation of resinous polymeric esters is prevented. Of course it is also understood that only those keto-acid esters which satisfy the aforementioned requirements of hygroscopicity, water-solubility and non-volatility will be useful to the invention.

As illustrative of specific materials which are useful in the practice of the invention, a number of compounds are listed in the following table together with data 'as to water-solubility and deformation, the latter figures being obtained when regenerated cellulose pellicles were impregnated as will be described in more detail below.

The compounds listed in the table are representative and it is to be noted that simple organic chemical compounds are effective for the purposes of the invention. All of the substances listed satisfy the criteria for operability previously discussed.

The impregnating agents of the invention may be introduced into cellulosic pellicles in the same way that glycerol is usually introduced in the manufacture of present-day commercial regenerated cellulose sheeting. Thus, a sheet of gel regenerated cellulose which has been purified and washed is impregnated with an aqueous glycerol solution, the excess of such solution removed by suitable means and the sheet dried. The commercial operation is continuous and the time of immersion in the glycerol bath varies with the speed of the machine although about 20 seconds immersion is probably normal. The concentration of the glycerol in the bath is adjusted to leave a predetermined amount of glycerol in the sheet after the drying operation, which latter removes all but about 6-8% water, based on the weight of the cellulose. Generally, the amount of glycerol in the final product is of the order of 15% and it has been found that a bath containing about 4-6% glycerol will give the desired results.

Accordingly, in the practice of the present invention the gel pellicle is impregnated with a bath which may contain conveniently about 46% of the non-deforming agent. After removal of excess bath, as by squeeze rolls and the removal of excess water by drying, the final product will contain a suitable amount of non-deforming agents Obviously, the amount of such agent in the final product will be adjusted to suit the desired properties expected in the final sheet so that the bath concentration may be more or less than 4% as occasion demands. It has been found however that the effectiveness of most non-deforming agents appears to approach a limiting value so that excessive amounts do not produce sufficient improvement to justify their use. Usually a bath concentration of about 4% will be found satisfactory for a realization of good non-deforming properties and at the same time economy of operation. Where the sheet is to be used at low temperatures and low humidities, a 6% solution may be found advantageous.

As a specific example, a gel sheet of regenerated cellulose may be impregnated .with a 4% aqueous solution of propylene glycol dilevulinate. The resultant sheet, after drying shows a deformation of about'2.0, whereas a similar sheet impregnated Both sheets contain approximately 14% of the impregnating agent. In this casethe product is flexible, durable, transparent and shows a reduction in deformation as compared with the .with glycerol shows a deformation of 4.0-4.4.

glycerin softened sheet amounting to approximately 50%.

In the above table a number of compounds has been indicated, each of which is capable of use in the production of a regenerated cellulose pellicle which will show a deformation of not more than 3.0. It is to be understood that the compounds mentioned are illustrative. The list given is by no means exhaustive and this disclosure is intended to embrace all organic compounds or mixtures of the class defined which have properties as previously set forth. These compounds may be used alone or in combination with each other.

The hygroscopicity of the compound chosen for use will determine, in large measure, the

amount of reduction in deformation which will be observed. Thus, compounds having a hygroscopicity of from 60 to 80 will show in most cases a deformation of the order of 2.7 to 3.0, while if the hygroscopicity is from 25 to 60, a deformation of the order of 2.3 to 3.0 will usually be obtained and if the hygroscopicity is from 1 to 25 a deformation of about 2.3 or less (usually of the order of 2.0 to 2.3) will be achieved in most instances. Thus, it can be seen that any variation in properties which may be desired in the finished product can be secured by proper selection of a non-deforming agent or non-deforming softener in accordance with the principles set forth herein.

The cellulosic pellicles obtainable by means of the present invention are particularly suited to wrapping purposes since the reduced deformation substantially eliminates warping, swelling, wrinkling and breakage. Similarly, the lamination of such pellicles to materials such as paper or fabric is facilitated since there is less tendency for the laminated product to curl, buckle or wrinkle. When stacks of sheets are stored, there is a lesser tendency for contiguous sheets to sticka common experience with glycerol-softened sheets.

Generally, the durability, particularly at low temperatures of C. or less, is improved. The process described is especially advantageous from an economic viewpoint inasmuch as it offers a method of producing this new type of cellulosic pellicle without alteration of present manufacturing equipment.

While it is preferred that the cellulosic pellicle be impregnated with a solution of the non-deforming agent while the pellicle is in the gel state, i. e., before it is dried and while it still contains a large amount of water, it is within the broad scope of the invention to impregnate a film which has been dried and then rewetted.

Although the invention has been described with specific reference to films of regenerated cellulose prepared by the viscose process since it is in this field that the invention is of greatest utility, it will be understood that the broad scope of the invention includes the treatment of other types of cellulose film cast from aqueous or aqueous alkaline solutions. Thus, the non-deforming agents referred to above may be used with ad- I vantage in the treatment of films produced from cuprammonium cellulose solutions, from aqueous alkaline solutions of glycol cellulose or cellulose glycolic acid, from aqueous alkaline solutions of lowly etherified cellulose, e. g. lowly etherified methyl cellulose or lowly etherified ethyl cellulose, or from aqueous alkaline solutions of lowly esterified cellulose, e. g. lowly esterified cellulose acetate.

Since the invention is capable of considerable variation and modification, any change from the above specific details and examples which conforms to the spirit of the invention is intended to be included within the scope of the claims.

I claim:

1. Water-sensitive film suitable for use as a wrapping tissue formed from an aqueous alkaline cellulosic solution and containing, as a deformation-restricting agent, a softening agent for said film comprising an ester of a keto-acid having a hygroscopicity of from 1 to 80, a boiling point of at least 136 C. at a pressure of 12 mm. of mer cury, and a solubility of at least 1% in water at 20 C., said ester being present in sufficient quantity to restrict the deformation of the film to a maximum of 3.0.

2. Water-sensitive film as defined in claim 1 characterized in that said ester is propylene glycol di-levulinate.

3. Water-sensitive film as defined in claim 1 characterized in that said ester is diethylene glycol (ii-levulinate.

4. Water-sensitive film as defined in claim 1 characterized in that said ester is butoxy-ethoxy ethyl levulinate.

5. Regenerated cellulose film suitable for use as a wrapping tissue containing, as a deformation-restricting agent, a softening agent for said film comprising an ester of a keto-acid having a hygroscopicity of from 1 to 80, a boiling point of at least 135 C. at a pressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 C., said ester being present in sufiicient quantity to restrict the deformation of the film to a maximum of 3.0.

6..Regenerated cellulose film as defined in claim 5 characterized in that said ester is propylene glycol di-levulinate 7. Regenerated cellulose film as defined in claim 5 characterized in that said ester is diethylene glycol di-levulinate.

8. Regenerated cellulose film as defined in claim 5 characterized in that said ester is butoxyethoxy ethyl levulinate.

9. A process for reducing'the deformation of water-sensitive film formed from an aqueous alkaline cellulosic solution which comprises impregnating said film with an aqueous solution containing a sufficient quantity of a deformationrestricting agent to restrict the deformation of the film to a maximum of 3.0, said deformationrestricting agent being a softener for said film and comprising an ester of a keto-acid having a hygroscopicity of from 1 to 80, a boiling point of at least 135 C. at a pressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 C.

10. A process for reducing the deformation of water-sensitive film as defined in claim 9 characterized in that said ester is propylene glycol di-levulinate.

11. A process for reducing the deformation of water-sensitive film as defined in claim 9 char acterized in that said ester is diethylene glycol di-levulinate.

12. A process for reducing the deformation of water-sensitive film as defined in claim -9 characterized in that said ester is butoxy-ethoxy ethyl levulinate.

13. A process for reducing the deformation of a regenerated cellulose film which comprises impregnating said film with an aqueous solution containing a suflicient quantity of a deformationrestricting agent to restrict the deformation of the film to a maximum of 3.0, said deformationrestricting agent being a softener for said film and comprising an ester of a keto-acid having a hygroscopicity of from 1 to -80, a boiling point of at least 135 C. at a pressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 C.

14. A process for reducing the deformation of regenerated cellulose film as defined in claim '13 characterized in that said ester is propylene glycol di-levulinate.

15. A process forrediming: the deformation of regenerated cellulose film asdeflned in claim 13 characterized in that said ester is'diethylene glycoi di-levulinate.

16. A process for reducingthedeformation of regenerated cellulose film as defined in claim 13 characterized in that said ester is butox'y-ethoxy ethyiievulinate.

HENRY S. ROTHROCK;

CERTIFICATE OF CORRECTION. Patent No. 2,'l'TO,8b O. August 29', 1959.

HENRY S. ROIHROCK.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, first column, line LL, for the word "vibration" read variation; line 11, for "slow- 1y" read lowly,- line 26, for "pellice" read pellicle; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 31st day of October, A. D 1959.

Henry Van Arsdale,

(s l) Acting Commissioner of Patents. 

